Microwave heating of energetic materials

ABSTRACT

Mixtures of high explosives with materials that readily absorb microwaves ignite more readily when exposed to microwave energy than the corresponding neat explosives. A charge of HMX (0.5 gram) mixed with carbon nanotubes (1 percent by mass) ignited with 7.5 joules at an average rate of 750 W for 10 milliseconds. To raise a charge of the same mass of neat HMX to an autoignition temperature of 200 degrees Celsius would require much more energy (about 110 joules) for a longer duration (about 150 milliseconds).

STATEMENT REGARDING FEDERAL RIGHTS

This invention was made with government support under Contract No.W-7405-ENG-36 awarded by the U.S. Department of Energy. The governmenthas certain rights in the invention.

FIELD OF THE INVENTION

The present invention relates generally to devices employing energeticmaterials (i.e. explosives) and more particularly to microwave heatingof a charge of explosive.

BACKGROUND OF THE INVENTION

Microwave radiation is electromagnetic radiation with a frequency in therange from about 1,000 MHz to about 30,000 MHz. Microwave techniqueshave been employed for a wide variety of applications that include radioastronomy, long distance communication, navigation, microwave ovens, andthe study of physical and chemical properties of matter.

Recently, the ignition of several important high explosives by microwaveirradiation has been reported (see Kazuo Hasue, Masami Tanabe, NobutuneWatanabe, Shoji Nakahara, and Fumiaki Okada, “Initiation of SomeEnergetic Materials by Microwave Heating,” Propellants, Explosives,Pyrotechnics, vol. 15, pp. 181-186 (1990), incorporated by referenceherein). Samples of high explosive (PETN, RDX, HMX, and TNT, forexample) were confined in tubes and ignited by microwave radiationhaving a frequency of 2450 MHz±50 MHz. The TABLE below the nextparagraph summarizes some of the properties of these explosives.

The uniformity of heating by microwave radiation is related to thedielectric properties of the material being heated. The absorbed power,which results in microwave dielectric heating, is given by equation (1)P=( 5/9)×f×E ²×ε×tan δ×10⁻¹⁰ [W/m ³]  (1)

where f is the frequency in Hertz (Hz), ε′ is the real part of therelative dielectric constant, tan δ is the dielectric loss, and E is theelectric field intensity in volts/cm. Values for ε′, tan δ, and ε″ aregiven in the TABLE for the high explosives PETN, RDX, HMX, and TNT. Ifthe frequency f and the electric field E are constant, then the absorbedpower P depends on ε′×tan δ, which is equal to ε″, which is theimaginary part of the relative dielectric constant; ε′ and tan δ″,however, may change with changes in temperature and frequency. TABLEMelting Ignition material ε′ tanδ ε″ point (° C.) temperature PETN 2.13.0 × 10⁻³ 6.3 × 10⁻³ 141.3 200 RDX 2.5 6.7 × 10⁻⁴ 1.7 × 10⁻³ 204.1 200HMX 2.4 2.9 × 10⁻⁵ 7.0 × 10⁻⁵ 276-277 241 TNT 2.0 1.2 × 10⁻⁴ 2.4 × 10⁻⁴80.75 330

The power reflectivity is given by equation (2)|Γ|²=(P _(o) ′/P _(o))×100[%]  (2)where |Γ|² is the power reflectivity, P_(o) (kW) is the incident powerand P_(o)′ (kW) is the reflected power. Hasue et al. calculated aninitiation energy, E_(i), using equation (3)E _(i)=(P _(o) −P _(o)′)×t[kJ]  (3)where t is the initiation delay time in seconds.

As the TABLE shows, TNT, PETN, RDX, and HMX exhibit low dielectric loss,i.e. they have a small value for tan δ. For these materials, theabsorption of microwave radiation was low, indicated by the small valueof ε″.

When these energetic materials are exposed to microwave energy, theirtemperatures increase as they absorb the microwaves until the exothermicreaction takes over and they ignite. It is believed that as microwavesare directed at a location of the explosive charge, the temperaturerises exponentially at that location, ignition occurs and the reactionspreads out until it consumes the entire charge. The reaction can rangein intensity from a non-violent burning to very violent thermalexplosion; this depends largely on the level of confinement of theexplosive; an initiated explosive contained in a strong, gas tightvessel will explode violently while in most circumstances, an unconfinedexplosive will simply burn.

According to Kazuo Hasue et al. “Initiation of Some Energetic Materialsby Microwave Heating,” Propellants, Explosives, Pyrotechnics, vol. 15,pp. 181-186 (1990), for all of the explosives tested, a delay inignition using microwaves was observed. The delays observed are asfollows: PETN 71 seconds, RDX 88 seconds, HMX 176 seconds, and TNT 100seconds. These ignition times are too long for a practical use usingmicrowave ignition.

If the delay time for microwave ignition for high explosives weresignificantly reduced, ignition of high explosives using microwaveenergy may be employed as an initiation mechanism for practical devices.Therefore, there remains a need for reducing the delay time for ignitionof high explosives for microwave energy to be useful as an initiationmechanism.

Therefore, an object of the invention is a method for microwave ignitionof high explosives with a reduced delay time.

Another object of the invention is a high explosive mixture sensitizedto ignition by microwave energy.

Additional objects, advantages and novel features of the invention willbe set forth in part in the description which follows, and in part willbecome apparent to those skilled in the art upon examination of thefollowing or may be learned by practice of the invention. The objectsand advantages of the invention may be realized and attained by means ofthe instrumentalities and combinations particularly pointed out in theappended claims.

SUMMARY OF THE INVENTION

In accordance with the purposes of the present invention, as embodiedand broadly described herein, the present invention includes a methodfor igniting high explosive. The method includes preparing a mixture ofhigh explosive having a first dielectric loss and a material having asecond dielectric loss that is higher than the first dielectric loss ofthe high explosive, and thereafter exposing the mixture to microwaveenergy.

The invention also includes a mixture of high explosive having a firstdielectric loss and about 1 percent by mass or less of a material havinga second dielectric loss that is higher than the first dielectric lossof the high explosive.

BRIEF DESCRIPTION OF THE DRAWINGS

The accompanying drawings, which are incorporated in and form a part ofthe specification, illustrate the embodiment(s) of the present inventionand, together with the description, serve to explain the principles ofthe invention. In the drawings:

FIG. 1 shows a schematic representation of an apparatus that was used todemonstrate microwave ignition according to the present invention.

FIG. 2 shows a graph of the absorbed power, and intensity of lightoutput, as a function of time for a sample of HMX (0.5 gram) mixed with1 percent by mass of carbon nanotubes.

FIG. 3 shows a graph of the absorbed microwave power, and absorbedenergy, as a function of time for the sample of FIG. 2.

FIG. 4 shows a photomacrograph of fragments generated by microwaveinitiation of a sample of HMX and carbon nanotubes; and

FIG. 5 shows a photo macrograph of fragments generated by microwaveinitiation of another sample of HMX and carbon nanotubes.

DETAILED DESCRIPTION

The invention relates to the ignition of explosives using microwaveenergy. For the purposes of this invention, the terms “energeticmaterial” and “explosive” are interchangeable. One aspect of theinvention involves preparing mixtures of high explosive that can beignited using much less energy than what would is required for the neathigh explosive. Another aspect of the invention relates to reducing thedelay for ignition. High explosives contemplated with this inventioninclude well-known high explosives such as PETN, RDX, HMX, TNT and TATB,and mixtures of these explosives. These high explosive materials do notabsorb microwave radiation readily and do not ignite promptly, aspreviously described (see K. Hasue et al. in “Initiation of SomeEnergetic Materials by Microwave Heating,” Propellants, Explosives,Pyrotechnics, vol. 15, pp. 181-186 (1990)). They are low loss dielectricmaterials and require ignition times that, according to K. House, varyfrom 71 seconds to 176 seconds. For this reason, devices and methodsemploying microwave ignition of these types of materials are notpractical.

According to the invention, high explosive is mixed with materials thatabsorb microwave radiation strongly. These mixtures can be ignited usingmicrowave energy, do not require as much microwave energy as the neatexplosive does for ignition, and have a much shorter ignition time thanthe neat explosive does.

The materials that can be mixed with high explosive to sensitize theexplosive to microwave radiation include carbon nanotubes, finelydivided metallic particles, and semiconductor particles. These areexamples of materials that absorb microwave energy strongly. It isexpected that any solid metal could be used. Only a very smallpercentage of the mixture, one percent by total mass and even less, isneeded to sensitize an explosive charge to microwave radiation. Exampleof metals may be useful with the invention include, but are not limitedto, aluminum, iron, and tungsten.

The preparation of mixtures of high explosive with carbon nanotubes andthe ignition of these mixtures using a flash of light has been describedin U.S. patent application Publication US2004/0040637. According to the'637 Application, energetic mixtures containing from 3 percent to 20percent by weight of carbon nanotubes with explosives (ammoniumperchlorate, RDX, TNT, and black powder) were prepared and ignited usinga light flash. This type of ignition is a surface ignition; the flashpromotes ignition on the surface and not throughout the bulk of thematerial because the light does not penetrate the material. By contrast,the present invention relates to volume ignition using microwaves. Themicrowaves penetrate the entire volume of the mixture, and microwaveradiation is absorbed by carbon nanotubes throughout the mixture. Volumeignition results in rapid consumption of the entire charge, not justignition and burning from the surface of the mixture.

The invention was demonstrated by preparing a mixture of HMX and a smallamount of carbon nanotubes. Only about 0.1% percent by mass of carbonnanotubes relative to the HMX produced a mixture with an ignition lessthan one tenth the time required for neat HMX.

In order to construct a practical explosive device, materials referredto herein as “sensitizers” were mixed with energetic material at lowconcentrations in order to significantly increase the local dielectricloss. When exposed to microwave energy, the sensitizer creates “hotspots,” and ignition occurs a many locations throughout the entirecharge of energetic material. Adding the sensitizer to the energeticmaterial also increases the overall dielectric constant.

Sensitizers of the present invention include, but are not limited to,carbon nanotubes, metallic and semiconductor particles. Lossy liquid,i.e. a liquid having a higher dielectric loss than that of the neatexplosive material (lossy liquids such as water and acetone, to name afew) or plastics, can also increase the overall dielectric loss of themixture.

It is believed that the sensitizer allows the quasi-uniform absorptionof microwaves such that the temperature of the energetic materialincreases uniformly, and/or that the sensitizer provides many locationsin the mixture for localized absorption, with induced ignition at thesewell-distributed discrete “hot spot” locations. If the absorbingsensitizer causes large thermal gradients in a small region, detonationmay occur. In any case, the presence of the sensitizer reduces theignition delay time to such an extent that microwave ignition in adevice employing this type of sensitized charge becomes practical.

Binders may be used with the invention. Useful binders include organicand/or inorganic materials that have a higher dielectric loss than thedielectric loss of the explosive. Some, or all, of the sensitizer couldbe the binder.

This technique allows for a wide range of energy release rates.Traditional detonating explosives release their energy very rapidly, ata steady rate, creating high peak pressures and a short pressure pulseduration. The characteristic blast from detonating explosives is veryeffective for destroying some types of structures, but improved couplingoccurs for many structures by lowering peak pressures and increasing theduration of the pressure pulse. In practice, it is difficult toaccomplish this with a device that has good timing control. Theinvention described here allows for the rapid release of explosiveenergy over a wide range of impulse characteristics in a single device.

Another aspect of the invention relates to a system that includes amicrowave source capable of producing microwave radiation and anyenergetic material that is sensitive to microwave ignition. Thesematerials can include classical CHNO explosives, insensitive highexplosives, high-nitrogen materials, thermites, metastableintermolecular composites, and the like. The energetic material may haveintrinsic high dielectric loss, or may include low dielectric lossexplosive in combination with sensitizers such as metallic particles,semiconducting particles, nanoscale and/or microscale fibers, includingcarbon nanotubes. The sensitizer may be distributed throughout theenergetic material uniformly or in such a way such that some desiredperformance characteristic is achieved.

The energetic material may contain more than one type of sensitizer,where each sensitizer may absorb microwave energy of some particularfrequency, or within a particular frequency range, or have a particularmicrowave power response, such that different frequencies or powerlevels induce different behaviors.

To facilitate extremely fast energy release rates, the energeticmaterial may be a composite consisting of various energetic materials,some sensitized and some not, again to provide flexibility inperformance. An example of this type of material may be a HMX/PETNcomposite, where the PETN contains the sensitizer (the HMX could beuniformly loaded with 100 micron PETN crystals that are uniformly loadedwith 0.1% carbon nanotubes, for example).

The common exploding-wire detonator uses PETN. A particulate mixture ofPETN crystals and sensitizer (carbon nanotubes or finely dividedmetallic or semiconductor particles, for example) would be employed as amicrodetonators that could be distributed in a bulk charge of adifferent explosive, say HMX. The distribution could be uniform, or havecharacteristics that would lead to some specific, desired performance.Using sensitized PETN in such a way would produce detonation waves frommany well-distributed locations throughout the composite.

The following EXAMPLES demonstrate the operability of the invention. TheEXAMPLES demonstrate the performance of a mixture of 0.5 g HMX and 1percent by mass carbon nanotubes when exposed to microwave radiation.For comparison, 0.5 g HMX exposed to the same or higher power microwavesover the same or longer duration did not ignite. The EXAMPLES wereperformed using a microwave initiation apparatus similar to thatdescribed by Kazuo Hasue, Masami Tanabe, Nobutune Watanabe, ShojiNakahara, and Fumiaki Okada, “Initiation of Some Energetic Materials byMicrowave Heating,” Propellants, Explosives, Pyrotechnics, vol. 15, pp.181-186 (1990). The used to demonstrate the invention included amicrowave generator, a microwave tuner, a power measurement apparatusand a shorted waveguide that provided a region of standing-wavemicrowave energy to facilitate the absorption of microwave energy intothe energetic material. The energetic material and associated fixturesperturb the impedance of the waveguide, and a three-stub tuner wasemployed to induce maximum power transfer to the energetic material.

A schematic representation of the apparatus is shown in FIG. 1.Apparatus 10 includes microwave generator 12, which supplied the 2.45GHz, 6 kWatt microwaves. Microwaves pass from generator 12 throughwaveguide 14, also known as the microwave applicator, to cylinder 16 andthen to the sample of energetic materials 18. Cylinder 16 was a 1-inchdiameter quartz cylinder that was used to contain the sample ofenergetic material 18 that was being exposed to the microwaves. Forwardand reflected powers are illustrated and were measured using directionalcouplers. With further regard to sample containment, a Pyrex tube (notshown) having an inner diameter of about 3/16 inch was used to confinethe sample. Cylinder 16 surrounded the Pyrex tube, protected thewaveguide and other components of the microwave initiation apparatus,and also acted as a “witness” for energy release rate by fragmentingduring the explosion event that followed ignition. The term “witness” isa commonly used term that is used herein to describe a passive objectthat is affected in some way by an explosive event. The witness was usedin order to provide some measure of the explosive event. In the EXAMPLEbelow, the quartz ‘witnesses’ the explosive event and providesinformation about it by how it fragments as a result of the explosion.

EXAMPLE 1

Ignition of a mixture of HMX with carbon nanotubes. A mixture of HMX(0.5 g) and carbon nanotubes (1 percent by mass) was prepared andignited by microwave radiation. FIG. 2 shows a graph of absorbed poweras a function of time (solid line, power in units of Watts, time inunits of milliseconds) of the mixture. FIG. 2 also shows the photodiodesignal (i.e. the light output) as a function of time (dashed line) fromthe mixture, along with the absorbed power. As FIG. 2 shows, themicrowave power was turned on at a time of minus 10 milliseconds, andignition occurred about 10 milliseconds later.

FIG. 3 shows a graph of the absorbed microwave power as a function oftime (solid line) and a graph of the absorbed energy as a function oftime (the dashed line) of the mixture. The absorbed energy is theintegral of the absorbed power). The total absorbed energy was 7.5Joules (J) at an average rate of 750 Watts for a duration of 10milliseconds. For comparison, raising the same mass of neat HMX to aconservative autoinitiation temperature of 200 degrees Celsius wouldrequire about 110 J for a duration of 150 milliseconds. The neat HMXwould also require much higher electric field strengths due to a weakerinteraction of neat HMX with microwave energy.

EXAMPLE 2

Evidence for variable energy release rate. The size of containerfragments generated during an explosive event provides a measure of theenergy release rate (see, for example, P. R. Lee, “Hazard Assessment ofExplosives and Propellants” in Explosive Effects and Applications, J. A.Zukas and W. P. Walters, Springer-Verlag (New York, 1998) p. 327,incorporated by reference). For this example, two experiments wereperformed to examine the fragment size of the pyrex sample container andthe quartz containment cylinder. In both experiments, the nominalconditions were the same: incident power, sample size and sample loadingwere nominally identical. Absorbed power data was not recorded butinconsistencies between the two experiments resulted in a difference inimpedance matching such that the absorbed power was different, which isindicated by the difference in sizes of the fragments in FIG. 4 and FIG.5. FIG. 5 (experiment 1) shows larger fragments than those of FIG. 4(experiment 2). A scale bar was not included in these FIGURES, but thepenny provides some indication of the fragment sizes. These qualitativeresults indicate that the energy release rate was significantly fasterin EXAMPLE 1 compared to EXAMPLE 2.

In summary, the present invention relates to igniting energeticmaterials using microwave radiation. High explosives were rendered moresensitive to microwave heating by the addition of sensitizers. Effectivesensitizers have a dielectric loss that ranges from one to severalorders of magnitude greater than that of the explosive. The addition ofthese lossy dielectric materials provides the user with the ability tobetter control the overall behavior of the subsequent explosive event,i.e. the explosion. The added sensitizers allow tuning of the energyrelease rate, up to detonation of the explosive. The added sensitizersalso allow for prompt ignition of energetic materials.

The foregoing description of the invention has been presented forpurposes of illustration and description and is not intended to beexhaustive or to limit the invention to the precise form disclosed, andobviously many modifications and variations are possible in light of theabove teaching.

The embodiment(s) were chosen and described in order to best explain theprinciples of the invention and its practical application to therebyenable others skilled in the art to best utilize the invention invarious embodiments and with various modifications as are suited to theparticular use contemplated. It is intended that the scope of theinvention be defined by the claims appended hereto.

1. A method for igniting high explosive comprising preparing a mixtureof high explosive having a first dielectric loss and a material having asecond dielectric loss that is higher than the first dielectric loss ofthe high explosive, and thereafter exposing the mixture to microwaveenergy.
 2. The method of claim 1, wherein the explosive is selected fromthe group consisting of TNT, PETN, RDX, HMX, TATB, and mixtures thereof.3. The method of claim 1, wherein the material that strongly absorbsmicrowave energy is selected from the group consisting of carbonnanotubes, metal powder, semiconductor powder, and mixtures thereof. 4.The method of claim 3, wherein the metal powder comprises finely dividedpowder.
 5. The method of claim 3, wherein the semiconductor powdercomprises finely divided powder.
 6. The method of claim 3, wherein themixture comprises about 1 percent by mass or less of carbon nanotubes.7. The method of claim 1, wherein the material that strongly absorbsmicrowave energy comprises a semiconductor.
 8. The method of claim 1,wherein the method further comprises adding a binder to the mixture ofhigh explosive having a first dielectric loss and a material having asecond dielectric loss that is higher than the first dielectric loss ofthe high explosive
 9. A mixture of high explosive having a firstdielectric loss and about 1 percent by mass or less of a material havinga second dielectric loss that is higher than the first dielectric lossof the high explosive.
 10. The mixture of claim 9, wherein the highexplosive is selected from the group consisting of TNT, PETN, RDX, HMX,TATB, and mixtures thereof.
 11. The explosive of claim 9, wherein thematerial that strongly absorbs microwave energy is selected from thegroup consisting of carbon nanotubes, metal powder, and semiconductorpowder.
 12. The explosive of claim 11, wherein the metal powdercomprises finely divided powder.
 13. The explosive of claim 11, whereinthe semiconductor powder comprises finely divided powder.
 14. Theexplosive of claim 9, further comprising a binder.